IconVault's Certificate Decoder reads X.509 certificates and CSRs with a hand-written ASN.1/DER parser, no dependencies, no uploads. Paste a PEM block or upload the file and get the subject and issuer, validity dates, serial number, signature algorithm, public key type and size, SANs, key usage and extensions in a clean readable view.

Everything is free and runs fully in your browser, which matters for certificates: pasting a cert into a random website is risky, so local-only decoding keeps your data on your machine. Honest note: it decodes structure only. It does not validate the trust chain or check live servers.